内蒙古东乌旗早二叠世超镁铁岩的发现及其构造意义

Discovery of the Early Permian Ultramafic Rock in Dong Ujimqi,Inner Mongolia and Its Tectonic Implications

为研究东乌旗晚古生代地幔性质和造山演化特征,对在东乌旗西部首次发现的超镁铁岩—辉闪橄榄岩进行系统的岩相学和元素地球化学研究。东乌旗辉闪橄榄岩主要由橄榄石、角闪石、斜方辉石及少量斜长石、单斜辉石组成,SiO_2含量介于42.84%~43.96%、MgO(24.10%~26.10%)、Na_2O+K_2O(1.52%~2.32%,小于3.5%),低m/f(3.03~3.54)比值和高FeO^T(12.67%~14.33%)的含量、高Mg~#(76.42~79.20),属铁质超镁铁岩和拉斑玄武岩系列。岩石稀土总量较高(∑REE=39.57×10^(-6)~83.32×10^(-6)),轻稀土(LREE)相对于重稀土(HREE)明显富集[(La/Yb)_N=4.04~7.66],Eu异常不明显(δEu=0.90~0.95),稀土元素球粒陨石标准化配分模式表现为轻稀土相对富集的右倾型。岩石富集大离子亲石元素(LILE)Cs、Rb、Ba、Sr、K等,相对亏损高场强元素(HFSE)Nb、Ta,具明显的Nb、Ta、Ti负异常,而又有别于强烈Nb、Ta亏损的岛弧岩浆岩;相容元素Cr(769×10^(-6)~2480×10^(-6))、Ni(454×10^(-6)~901×10^(-6))含量较高;低Th/U(2.54~3.03)、Nb/U(3.40~12.85)比值和高La/Nb(2.60~3.63)、Ba/Nb(43.11~72.52)、Zr/Y(5.45~7.83)比值。综上,结合区域最新研究成果,我们认为辉闪橄榄岩来源于受俯冲流体交代的尖晶石相地幔橄榄岩部分熔融,上升过程中受地壳物质不同程度的混染,形成于早二叠世板内伸展构造体制,与古亚洲洋闭合之后板内非造山作用有关。这一认识填补了早二叠世幔源岩浆事件和非造山阶段超镁铁质岩石记录的空白。

This study carried out petrographical and geochemical analyses for the ultramafic rock （i. e. schriesheimite） firstly recognized in the west of Dong Ujimqi so as to understand the characteristics of the late Paleozoic mantle nature and structure in Dong Ujimqi. Schriesheimite consists mainly of olivine, hornblende, orthopyroxene, with minor amount of slight plagioclase and clinopyroxene. Geochemical research indicates that the schriesheimites have SiO2 （42. 84 % ～ 43. 96 %）, MgO （24. 10 % ～ 26. 10 %）, Na2O＋K2O（1.52%～2.32%, 〈3.5%）, and are higher in FeOT（12.67%～14. 33%）, Mg# （76.42～79. 20） and low in m/f（3.03～3.54）. These features suggest the rock should belong to ferruginous ultramafic rock and tholeiitic series. The rocks are characterized by high REE （39. 57 × 10-6 ～ 83. 32 × 10-6） , enrichment in LREE and relatively depletion in HREE V（La/Yb）N=4.04～7.66], slightly Eu anomalies （δEu=0.90～0.95）, with the high La/Nb （2.60～3. 63）, Ba/Nb （43.11～72.52） and Zr/Y （5.45～7. 83）, and low Th/U （2.54～3.03） and Nb/U （3.40～12.85） which similar to intraplate rocks from mantle magmatism. The rocks are enriched in LILE （Cs, Rb, Ba, Sr, K）, and depleted in HFSE （Nb, Ta）, with distinct negative anomalies of Nb, Ta and Ti which are clearly different from the strong depleted of Nb and Ta of the island arc magmatic rocks. The rocks also have high compatible elements, such as Cr （769 × 10-6 ～2480× 10-6） and Ni （454× 10 6 ～901 × 10-6）, which are higher than proto-magma. In combination to the latest research results in the area, we consider that the schriesheimites in western Dong Ujimqi resulted from the partial melting of the Sp-peridotite in extensional tectonic background, which was replaced by subducted slab-derived fluid, along with various-degree mixing from the crustal material during its ascending, s from partial melting of the Sp-peridotite, which was modified by the the Early Permian subducted slab-derived fluid. The extensional tectonic background is related to within-plate anorogenic magamatism after the closure of Paleoasian Ocean. This recognition on the schriesheimites provides a record for existence of the Early Permian mantle-derived magmatic events and anorogenic ultramaifc rocks.